Computer users who still primarily work in networks require that the access to their data and resources in general is granted only to those that they allow to - just as in the case of physical property, the users of computer systems want the so-called computer security. The Internet, as the best known computer network, connects millions of people around the world granting them access primarily to a large amount of information and its users need to have the necessary means in order to achieve a given level of security. Systems for detecting intrusion in a computer system (IDS-instrusion detection system), solve the problem of unwanted network access. There are open-source and closed-commercial code IDS and it is important to have an insight into their advantages and disadvantages.